1. Field of the Invention
The present invention relates to a vacuum valve arranged to open and close while bellows is contracted and elongated.
2. Description of Related Art
Heretofore, for example, in a CVD device of a semiconductor manufacturing system, a material gas made of constituents for a thin-film material is supplied to a wafer in a reaction chamber, which is a vacuum container, through an inlet port thereof. Simultaneously, the reaction chamber is evacuated of the gas by a vacuum pump through an outlet port of the reaction chamber so that the reaction chamber is maintained under vacuum. A discharge speed of the material gas is controlled by for example a butterfly proportion valve. However, such butterfly proportion valve could not completely block a pipe. Accordingly, an ON/OFF vacuum valve is disposed in series with the butterfly proportion valve for completely interrupting the flow of a fluid in the pipe.
One example of the vacuum valve is disclosed in JP2003-83467A. This vacuum valve is shown as a sectional view in FIG. 15.
A vacuum valve 100 in FIG. 15 is operated to open and close by applying a driving force of an actuator section 102 to a valve section 101. In the actuator section 102, a piston 103 slidably installed in a cylinder 110 is coupled to one end of an output shaft 104 placed extending through a boss 111 of the cylinder 110. The output shaft 104 is thus guided axially. The other end of the output shaft 104 protrudes from the boss 111 into the valve section 101.
In the valve section 101, a valve element 105 is fixedly attached to the end of the output shaft 104 protruding in a valve body 112, and a valve seat 107 with which the valve element 105 is moved into or out of contact is provided between a first port 113 and a second port 114 of the valve body 112. The valve element 105 is always urged toward the valve seat 107 by a compression spring 115. A bellows 108 made of metal is placed in the valve body 112 in such a manner as to surround the compression spring 115 and the output shaft 104 and be elastically elongated and contracted.
The above vacuum valve 100 is operated in such a way that, when the piston chamber 109 is pressurized to move the piston 103 upward in the figure against the urging force of the compression spring 115, the valve element 105 is moved away from the valve seat 107, thereby allowing communication between the first port 113 and the second port 114.
When pressurizing of the piston chamber 109 is stopped, on the other hand, the piston 103 is moved downward by the urging force of the compression spring 115, thereby bringing the valve element 105 into contact with the valve seat 107 to completely close off fluid communication between the first port 113 and the second port 114.
While the above valve opening and closing operation is performed, the bellows 108 is contracted and elongated as the valve element 105 is moved and functions to hermetically separate the inside of the bellows 108 from the outside thereof. Accordingly, particles which may be generated in a sliding portion or the like of the output shaft 104 is unlikely to flow in the flow passage.
The conventional vacuum valve 100 however has disadvantages that the bellows 108 tends to be damaged or broken after long use, leading to leakage of the particles in the flow passage. Such damage of the bellows 108 has been regarded heretofore as being caused by impact or shock of the valve element 105 with the valve seat 107 at the time of valve closing.
The inventors of the present application repeatedly tested many bellows 108 for endurance and found that most of the bellows 108 were broken due to cracks caused at given portions. The inventors further found that the bellows 108 were cracked on impact caused when the valve element 105 struck against the boss 111 of the cylinder 110 at the time of valve opening, causing breakage of the bellows 108.
Consequently, the inventors have conceived the causes of the above defects as follows.
Specifically, the bellows 108 is held in a floating manner within the valve section 101 so as to be elastically contracted and elongated according to movements of the valve element 105. In this state, when the valve element 105 moves upward to a stroke end and strikes against the lower end of the boss 111, that impact is transmitted to the bellows 108 via the cylinder 110 and simultaneously transmitted to the bellows 108 from the valve element 105. Such two impacts transmitted to the bellows 108 act on the bellows 108 in opposite directions to cause the bellows 108 to vibrate in different directions. It is thus conceivable that the internal stresses of the bellows 108 induced by the vibration interfere with each other at a given portion, causing stress concentration on the interference portion. The stress concentration is caused in the same portion of the bellows 108 every time the valve element 105 strikes against a pipe member 106, and hence the given portion of the bellows 108 is damaged faster than other portions.
In view of the above causes, it is conceivably possible to enhance the endurance of the bellows 108 by controlling the “operating speed” of the valve element 105 when it starts a valve opening operation and the “impact acceleration” transmitted to the bellows 108 in a contracted state when the valve element 105 strikes against the boss 111. Thus, the inventors further performed life tests of the vacuum valve 100 by using different “operating speed” and “impact acceleration”.
The life tests were executed on a plurality of vacuum valves having identical bellows and identical internal configurations by repeating a predetermined number of valve opening and closing operations of each vacuum valve and checking whether each valve reached a target number of operations corresponding to an expected useful life without damage of the bellows 108. The “operating speed” was measured by a speed sensor attached to the inner wall of the valve body 112. The “impact acceleration” was measured by an acceleration sensor attached to the valve element 105. Herein, the life tests were implemented on nineteen types of vacuum valves having different bellows and different internal configurations, in which each type includes a plurality of vacuum valves having identical bellows and identical internal configurations. The measurement results are shown in FIG. 16, in which some of the nineteen types of vacuum valves have the same results and hence nine marks represent the results of nineteen types of vacuum valves.
In FIG. 16, if a line is drawn between the long-life valves (determined in the life tests that the valves could reach the desired number of operations) and the short-life valves (determined in the life tests that the valves could not reach the desired number of operations), a hatched scope S in FIG. 16 is defined.
From the test results, as for the vacuum valves provided with the metal bellows, it is found that the bellows 108 were unlikely to be cracked in the case where the “operating speed” is 30 mm/s or more and 220 mm/s or less and the “impact acceleration” is in the hatched scope S in FIG. 16. If the operating speed exceeds 220 mm/s, increasing the impact acceleration accordingly, the bellows 108 is likely to be cracked. In this view, it is conceivable that the bellows 108 is unlikely to be cracked if the operating speed is reduced, decreasing the impact acceleration. In fact, however, even where the operating speed was lower than 30 mm/s and the impact acceleration was low, the bellows 108 was cracked as indicated in FIG. 16.
From the above test results, the inventors found that the bellows 108 of the vacuum valves 100 having the same stroke were decreased in endurance in both cases where the operating speed was too low and too high. Further, the inventors found that the endurance of the bellows 108 could be increased when the impact accelerator speed could be reduced even when the operating speed was high. Accordingly, if only the vacuum valve 100 is designed to increase the operating speed while reducing the impact acceleration, it is possible to lengthen the useful life of the bellows 108 to improve the endurance of the vacuum valve 100.
Some vacuum valves are arranged to allow the piston to abut against the inner wall of the piston chamber. As for this type of vacuum valve, similarly, the inventors also found from the endurance tests that the bellows 108 was cracked and broken as in the above case. Consequently, this type of vacuum valve has only to be designed to achieve a higher operating speed and a lower impact acceleration in order to increase the life of the bellows 108 and hence improve the endurance of the vacuum valve 100.